Does anyone have experience with acceleration for a kid with a 140-ish IQ, which I suppose is smart but not absurdly/scarily smart?
No, that really is unusually smart

. My oldest is grade accelerated and she's no brighter than that. She's a straight A student who still tests in the top 1-2% of the grade nationally and locally on grade level tests and above level tests. We're 3.5 yrs post skip now so I'd say that I can reasonally call it successful.
143 should be around the 99.7th percentile if I recall correctly -- maybe 99.8th. That is well out of the norm. General contentment and not appearing to need harder work on the surface isn't reason not to accelerate, though. A good reason might be a disability standing in the way such that the child would struggle post-skip even with a very high IQ.